1. Field of the Invention
This invention relates generally to the field of high-voltage power supplies for video display apparatus, and, in particular, to the field of regulating voltages developed for cathode ray tubes used in video display apparatus, such as televisions, computer monitors, and the like.
2. Background Information
An electron gun in a cathode ray tube generates an electron beam that is used to scan information onto the screen of the cathode ray tube. The electron gun utilizes a focus voltage applied to a focus grid to adjust the diameter, also referred to as the "spot" or "spot size," of the electron beam. After being focused by the focus voltage at the focus grid, the electron beam is accelerated toward the screen of the cathode ray tube by the ultor voltage, also referred to as the anode voltage. The ultor voltage is applied to the cathode ray tube at the anode button, which is located on the bell-shaped portion of the cathode ray tube.
The ultor voltage in a video display apparatus is typically generated by the horizontal deflection system, which comprises a horizontal deflection circuit and a flyback transformer. Such a horizontal deflection system is conventional and will not be described further. In this approach, a horizontal retrace pulse voltage generated by the horizontal deflection circuit during its retrace mode of operation is applied to a primary winding of the flyback transformer. The horizontal retrace pulse voltage is stepped up by a high-voltage winding of the flyback transformer, and this stepped-up voltage is rectified and then filtered to provide the ultor voltage. The filtering is performed by an ultor capacitance, which may be provided by the capacitance formed between the inner and outer conductive coatings of the cathode ray tube.
An alternative approach to generating the ultor voltage is to utilize a dedicated high-voltage power supply. For example, in a video display apparatus that can support a range of horizontal scanning frequencies, it may be advantageous, for reasons relating to the complexity of the circuit design and the cost of materials, to utilize a separate high-voltage power supply to generate the ultor voltage. An example of a flyback type high-voltage power supply that may be used in video display apparatus is disclosed in U.S. Pat. No. 4,531,181. entitled HIGH VOLTAGE POWER SUPPLY and issued to Herz et al. The ultor voltage at the output of the high-voltage power supply is filtered by an ultor capacitance, which may be provided by the capacitance formed between the inner and outer conductive coatings of the cathode ray tube.
The high-voltage power supply disclosed in U.S. Pat. No. 4,531,181 utilizes negative feedback to regulate the ultor voltage. A resistor divider network divides the ultor voltage to provide a feedback signal that varies in proportion to changes in the level of the ultor voltage. This feedback signal is used to control a device which regulates the B+ input voltage to the high-voltage power supply. Thus, if the ultor voltage decreases in response to a higher beam current being drawn by the cathode ray tube, the B+ input voltage is increased, thereby increasing the ultor voltage. Conversely, if the ultor voltage increases in response to a lower beam current being drawn by the cathode ray tube, the B+ input voltage is decreased, thereby decreasing the ultor voltage.
A grid voltage, such as a focus voltage or a screen voltage, can then be generated from a so-called "focus screen" assembly, which is energized by the high-voltage winding of the flyback transformer to generate the focus and screen voltages for the cathode ray tube. The focus screen assembly may include a network of fixed resistors, variable resistors, and capacitors. A resistor chain of the focus screen assembly generates the required focus and screen voltages for the cathode ray tube. The component resistors of the focus screen assembly are deposited on a ceramic substrate, and the assembly is fully enclosed and insulated. The means for adjusting the variable resistors to set the screen and focus voltages are accessible from outside the case of the focus screen assembly.
U.S. Pat. No. 5,602,447, entitled CATHODE RAY TUBE FOCUS SUPPLY and issued to Smith, discloses three approaches for energizing a focus screen assembly: the resistor divider network approach, the peak detected approach, and an inventive combination of the resistor divider network and peak detected approaches. The high-voltage power supply disclosed in U.S. Pat. No. 4,531,181 uses the resistor divider network approach. This approach, shown in FIG. 1, entails energizing a plurality of series-connected resistances with the full ultor voltage. Some of the plurality of resistances may comprise variable resistances. The required focus and screen voltages are then set by adjusting these variable resistances.
The peak detected approach might also be used to generate the desired grid voltage. This approach, shown in FIG. 2, entails energizing a plurality of series-connected resistances from a tap of the high-voltage winding of the flyback transformer. For example, the tap for the focus voltage is typically chosen so that the focus voltage is approximately one-third of the ultor voltage;
in other words, the "focus ratio" is equal to approximately one-third. The voltage at the tap should be greater than the required focus voltage, so that an adjustment range is available. Again, some of the plurality of resistances may comprise variable resistances, and the required focus and screen voltages are then set by adjusting these variable resistances.
It has been empirically determined that, when the peak detected approach is used to generate voltages in a dedicated high-voltage power supply, the ultor capacitance may adversely affect the regulation of those voltages at the lower end of the range of beam current conditions. Specifically, at lower levels of screen brightness, and hence lower levels of beam current, the focus voltage may have a tendency to drop below an acceptable minimum level, resulting in artifacts on the screen of the cathode ray tube caused by spot defocusing of the electron beams. In addition, at such lower levels of beam current, the screen voltage may also tend to drop, thus resulting in a shift of the cathode ray tube's cutoff voltage, also referred to as its "black level."
For the foregoing reasons, there is a need for a high-voltage power supply that provides regulation of the voltages generated by the peak-detected approach to ensure that these voltages do not drop below a predetermined minimum level.